An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors, for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor.
Backside illuminated sensors are used for sensing a volume of exposed light projected towards the backside surface of a substrate. The pixels are located on a front side of the substrate, and the substrate is thin enough so that light projected towards the backside of the substrate can reach the pixels. Backside illuminated sensors provide a high fill factor and reduced destructive interference, as compared to front-side illuminated sensors.
A problem with backside illuminated sensors is that the substrate is relatively thin to allow for sufficient radiation transmission. To accommodate the thinned substrate, it is typical to attach a supplemental device, such as a carrier wafer, to the front side of the wafer (over the metal lines and other backend layers) on which one or more sensors are being fabricated. Typically, the carrier wafer is a semiconductor material, such as silicon, or an optical transparent material, such as glass, which has a thickness of greater than 200 μm. However, the carrier wafer blocks or prevents ready access to the bond pads of individual sensors on the wafer. As a result, certain types of wafer acceptance testing cannot be performed, such as probe testing and design performance testing. Probe testing measures performance of device or process parameter, such as transistor performance, line spacing, and line, to ensure that the values are within specification. Design performance testing checks the functionality of the chip to ensure that it is within design performance specification.
A need exists for a method and system that allows wafer level testing to be performed on backside illuminated sensors without passing through the thick carrier wafer.